Brake system for automotive vehicles with controllable brake force distribution

ABSTRACT

A brake system provided for automotive vehicles comprises a braking pressure generator (3) and a braking pressure modulator (4) which is arranged in the pressure medium conduit leading to the rear wheel brakes (16, 17) and which allows to vary the apportioning of the braking pressure to the wheel brakes of the front and the rear axle to achieve approximation to an ideal brake force distribution in dependence on the instantaneous axle load. The braking pressure modulator (4) contains valve assemblies (5) serving to supply braking pressure alternately to the wheel brakes (16, 17) of the right and the left rear wheel in such a manner that one of the two rear wheels (HR or HL) furnishes the predominant portion of the brake force allotted to the rear axle. The pressure delivery alternates to connect to the other rear wheel on each braking operation or on each brake actuation or according to predetermined time criteria.

BACKGROUND OF THE INVENTION

The present invention relates to a brake system for automotive vehiclescomprising a braking pressure generator and a controllable brakingpressure modulator which is arranged in the pressure medium conduitsleading to the rear-wheel brakes. The system allows variation of theapportioning of the braking pressure onto the wheel brakes of the frontaxle and the rear axle for the purpose of approximation to an idealbrake force distribution dependent upon the instantaneous axle load.

It is known that the dimensioning of brake systems is rendered difficultby the changes in static and dynamic axle loads and by axle load shifts.Therefore, by invariably adjusting the braking pressure distributiononto the front axle and the rear axle, optimum conditions may beaccomplished at most in a specific state of loading and driving.

Moreover, for decreasing the imminent danger of skidding, the share inbraking pressure allotted to the rear-wheel brakes is desired to be assmall as to allow the rear wheels to lock in all situations only afterthe front wheels do, although the rear wheels are relieved from loadowing to the dynamic axle load shift when braking at higher speed and,in consequence thereof, tend to lock sooner than the front wheels. Forthis reason, a considerably larger portion of braking pressure must beallotted to the front axle when brake force distribution is invariable.

A certain improvement or, respectively, approximation to the ideal brakeforce distribution permits to be obtained by the known brake forcedistributors which vary the brake force distribution onto the front andthe rear axle according to various function principles in response tobraking pressure, load or deceleration. The extent of adaptation to theideal distribution attainable differs widely. In general, even whensophisticated devices and great adjusting efforts are involved,satisfactory adaptation to the ideal characteristic curve can beobtained at most in either one of the two borderline cases"unloaded/loaded".

A brake force distributor is known in the prior art wherein the staticaxle load distribution is measured by sensors when the vehicle is atstandstill. The results thereof are delivered to a microcomputer whichgoverns the brake force distribution, in consideration of the testvalues, according to a memorized mathematical relation and whileadditionally considering the pressure measured in the front-axle circuitand in the rear axle circuit (European publication EP-AL No. 062246).Despite the high effort entailed, in this device the adaptation is alsofar from being ideal in the majority of driving situations, because onlya theoretical adherence value, but not the actual value of adherencebetween road and tires can be made the basis for dimensioning thebrakes, and for reasons of safety and for prevention of overbraking ofthe rear wheels, the device must be dimensioned such that the front axlesupplies the greater contribution to braking.

It has been proposed to have electromagnetically controlled modulatorscontrol the brake slip at the rear wheels in dependence on the brakeslip at the front wheels such that a value of adherence will result atthe rear axle which is always equal to, or somewhat less than, that atthe front wheels (German publication No. P 33 01 948).

It is an object of the present invention to overcome the disadvantgagesdescribed and to create a brake system which permits to attain a betterapproximation to the ideal brake force distribution or, respectively,brake force apportioning, onto the front and the rear axle withoutimpairing the directional stability of the vehicle and, respectively,without increasing the imminent danger of skidding (due to locking rearwheels).

SUMMARY OF THE INVENTION

This object is achieved in a surprisingly simple manner by improvingupon a brake system of the type initially referred to such that thebraking pressure modulator contains valve assemblies which allow tosupply braking pressure alternately to the wheel brakes of the right andof the left rear wheel such that, alternately, one of the two rearwheels furnishes the predominant share or all of the brake forceapportioned to the rear axle, that means that portion of brake forcethat is allotted to the rear axle in the event of roughly ideal brakeforce distribution.

The present invention is based on the fact that a short stoppingdistance can only be obtained by an optimally high utilization of thecoefficient of friction between tires and road and that it must beprevented that the two rear wheels lock in advance of the two frontwheels, in order to obtain sufficient lateral guiding force at the rearaxle and to thereby ensure directional stability during braking. Whileall presently known and used brake systems and brake force distributorsare designed such that, in the normal case as long as the system isintact, at least roughly the same braking pressure is acting in eachcase at both wheels of an axle, the present invention makes use of theconsideration that the driving stability of the vehicle will still besafeguarded, even in the event of one rear wheel locking, if the secondrear wheel continues to roll in an unbraked or slightly braked fashion.

In connection with the failure of one brake circuit in systems withdiagonal or right/left allotment of brake circuits, it has been provedmathematically and tested practically that driving stability of thevehicle will be maintained even in this exceptional situation when arear wheel locks.

Depending on the respective embodiment of this invention, achanging-over of the pressure delivery to the other rear wheel isperformed with every braking action or every brake actuation. It islikewise possible, on the other hand, to perform this change-over onlyafter a predetermined number of braking actions, according to a timecriterium or after having covered a specific driving distance, thechange-over being preferably effected when the brake is released. Forsafe-guarding even wear, this alternation from the one to the otherrear-wheel brake could also be made dependent on the brake pad thicknessremaining.

In one embodiment of this invention, only one nof the two rear-wheelbrakes, respectively, will be supplied with pressure during each brakingaction. On the other hand, it is of advantage in further embodiments if,until attainment of a predetermined limit value of braking, of thebraking pressure or of a derived braking value, both rear-wheel brakescan be acted upon by the same braking pressure, and if in excess of thislimit value the braking pressure remains constant in one of the tworear-wheel brakes, alternately, or rises but slightly in comparison tothe other rear-wheel brake. Said limit value can be varying independence upon the deceleration of the vehicle, of the axle loadsand/or in response to measured variables derived.

The braking pressure modulator of the inventive braked system issubstantially composed of one or more controllable devices, for example,electromagnetically or hydraulically actuatable multi-directionalvalves. A four-way/two-position directional control valve isparticularly apt as a valve assembly for the alternating pressuredelivery into the right and the left rear-wheel brake, the said valveopening in both of its switch positions each one pressure medium conduitfrom the braking pressure generator to one of the two wheel brakes ofthe rear axle, while it closes the pressure medium conduit to the otherrear-wheel brake or throttles its flow.

Instead of the above, if a four-way/three-position directional controlvalve is used, there will be available a third switch position in whichthe passage to both rear-wheel brakes is suitably closed. By dosedlyconnecting or, respectively, opening the flow passage, a like valveenables to develop the braking pressure in a measured quantity or,respectively, to sensitively control the braking pressure variation, forinstance in response to the braking pressure or the brake slip at thefront axle.

The brake system of the present invention is realized by minimalmanufacturing effort, while nonetheless attaining considerableimprovement of the brake force distribution. Moreover, the rear wheel isbraked minimally or considerably less and therefore is rolling almostwithout slip which permits in addition a direct measurement of theactual vehicle speed and deceleration, whereby the formation of avehicle reference value for the control of brake force distribution isconsiderably simplified.

BRIEF DESCRIPTION OF THE DRAWING

Further features, advantages and applications of this invention can begathered from the following description of an embodiment of thisinvention with reference to the accompanying drawing in which the singleFIGURE illustrates a brake system in accordance with the presentinvention.

DETAILED DESCRIPTION

In this schematically illustrated brake system, the front and the rearwheels communicate by way of two diagonal pressure medium circuits 1 and2 with a braking pressure generator 3, which may consist e.g. of atandem master cylinder in conjunction with a vacuum booster. The forceapplied via a pedal on the braking pressure generator 3 is symbolized bythe arrow F. The two front wheels VL, VR communicate directly with thebraking pressure generator 3, while the rear wheels HL, HR are connectedthereto via a braking pressure modulator 4.

In this example, the braking pressure modulator 4 is composed of threemulti-directional control valves 5, 6, 7. Said valves, shown in theirinactive position, are electromagnetically actuatable and are controlledby an electronics 8 via the signal lines 9, 10, 11. The signal in eachof lines 9, 10, and 11 is produced in a known fashion when the input ofthe electronics 8 is in turn supplied with measured signals which aregenerated by means of the sensors S₁ to S₄ and which transmit data aboutthe rotational behavior of the individual vehicle wheels. Herein,inductive pick-ups for measuring data are concerned in which, with theaid of toothed disc 18 to 21 co-rotating with the wheels, voltages areinduced whose frequencies are proportional to the rotating movement ofthe wheels. Besides, said electronics comprises further inputs 13 towhich there can be connected e.g. sensors for the hydraulic pressure inthe brake circuits 1, 2, or time elements for the determination of thechange-over times of the valve 5 etc.

In the embodiment shown herein, only in one diagonal 1 or 2, brakingpressure is introduced into the wheel brake 16 or 17 of the rear wheelHL or HR during each braking action by means of thefour-way/two-position directional control valve 5. The embodimentillustrated the drawing shows directional control valve 5 in itsinactive position, whereby hydraulic pressure fluid flows from circuit 1through valve port 22 to rear wheel brake 17 and fluid from circuit 2 iscut off from wheel brake 16. Upon actuation of directional control valve5 responsive to signals transmitted by electronics 8, the valve moves tothe left, as illustrated, whereby pressure fluid from circuit 1 to wheelbrake 17 is cut off, and pressure fluid in circuit 2 is connected towheel brake 16 by means of valve port 23. As pressure fluid is suppliedto either one of wheel brakes 16,17, the remaining unbraked rear wheelcontinues to run unbraked.

The delivery of braking pressure into the wheel brake 16 or 17 of thereal wheel will, however, become possible only after thetwo-way/two-position directional control valve 6, 7 arranged in serieshas been switched from its inactive position illustrated to assume itsopened position. This is because the brake system described hereinconcerns a device in which the brake force distribution is controlled independence upon the brake slip on the front wheels VL, VR such that atthe rear wheel HR, HL connected--depending on the switch position of thevalve 5--there will always result approximately the same or a somewhatlower value of adherence, as has been depicted in German publication No.P 33 01 948.

Another variant compared to the brake system illustrated or,respectively, to the braking modulator 4 consists in that under certainconditions, e.g. until attainment of a braking pressure limit value,there is first admitted braking pressure rise in the rear-wheel brakes16,17 of both rear wheels HL, HR, by use of additional valves or anadditional switch position of the valve 5, and in that the inventivevalve 5 is caused to operate only after further pressure rise, or,respectively, in that further braking pressure rise at the rear axle islimited to one of the two wheels HL or HR. In the second rear-wheelbrake, the braking pressure is maintained constant until termination ofthe braking action in this embodiment (not shown). The electronics 8 maylikewise be realized by means of a hard-wired logic or by programmedmicrocomputers.

What is claimed is:
 1. A brake system for automotive vehicles comprisinga braking pressure generator and a controllable braking pressuremodulator which is arranged in the pressure medium conduits leading toboth rear-wheel brakes and which allows variation of the apportioning ofthe braking pressure to the wheel brakes of the front axle and the rearaxle for the purpose of approximation to an ideal brake forcedistribution, wherein the braking pressure modulator (4) comprises valveassemblies (5) which upon the initiation of braking pressure allowbraking pressure alternately to the wheel brake of the right rear wheel(HR) and the left rear wheel (HL) so that one of the two rear wheelsfurnishes all of the brake force apportioned to the rear axle, which isthat portion of brake force that is allotted to the rear axle in theevent of approximately ideal brake force distribution.
 2. A brake systemas claimed in claim 1, wherein the pressure delivery alternates to thewheel brake (16, 17) of the other rear wheel (HL, HR) on each brakingoperation.
 3. A brake system as claimed in claim 1, wherein the pressuredelivery alternates to the wheel brake (16, 17) of the other rear wheel(HL, HR) after a predetermined number of braking operations or brakeactuations.
 4. A brake system as claimed in claim 1, wherein thepressure delivery into the wheel brakes (16,17) of the rear axlealternates to the other rear-wheel brake on termination of apredetermined period of time, while the said change-over is to beperformed when the brake is not actuated.
 5. A brake system forautomotive vehicles comprising a braking pressure generator and acontrollable braking pressure modulator which is arranged in thepressure medium conduits leading to both rear-wheel brakes and whichallows variation of the apportioning of the braking pressure to thewheel brakes of the front axle and the rear axle for the purpose ofapproximation to an ideal brake force distribution, wherein the brakingpressure modulator (4) contains valve assemblies (5) which upon theinitiation of braking pressure allow braking pressure alternately to thewheel brake of the right rear wheel (HR) and the left rear wheel (HL) sothat one of the two rear wheels furnishes the predominant share of thebrake force apportioned to the rear axle, which is that portion of brakeforce that is allotted to the rear axle in the event of approximatelyideal brake force distribution, wherein until attainment of apredetermined limit value of one of slowing down of the braking pressureand a derived measured variable, both rear-wheel brakes (16,17) areacted upon by the same braking pressure, and wherein, in excess of thislimit value, the braking pressure remains constant in one of the tworear-wheel brakes alternately.
 6. A brake system as claimed in claim 5,wherein the limit value is varied in dependence upon one of the vehicledeceleration and the axle load.
 7. A brake system as claimed in claim 5,wherein the braking pressure modulator (4) is composed substantially ofat least one controllable multi-directional control valve (5, 6, 7). 8.A brake system as claimed in claim 7, wherein a four-way/two-positiondirectional control valve is provided as a valve assembly (5) for thealternating pressure delivery into the right and left rear-wheel brake,said valve opening in both of its switch positions the passage of eachone pressure medium conduit leading from the braking pressure generator(1) to the wheel brake (16, 17) of one rear wheel (HL or HR), while itcloses the pressure medium conduit to the other rear-wheel brake.
 9. Abrake system as claimed in claim 7, wherein a four-way/two-positiondirectional control valve is provided as a valve assembly (5) for thealternating pressure delivery into the right and the left rear-wheelbrake (16, 17), said valve releasing in both of its switch positions oneunthrottled and one throttled passage to the two rear-wheel brakes (16,17), respectively.
 10. A brake system as claimed in claim 9, wherein themulti-directional control valves (5, 6, 7) of the braking pressuremodulator (4) are electromagnetically actuatable.